I have used a relatively powerful electric heater for my 2016 Toyota RAV4 hybrid. I believe the power is 700 watts. It is not an old-style block heater but not one of those that you attach to the side of the engine either. It actually heats the coolant with a circulating pump.
Even if I use the coolant heater for 2 hours, when starting the engine, the temperature gauge is still in the lowest point. Basically, the temperature gauge of a car is not designed to say whether the coolant is 0 degrees Celsius or 20 degrees Celsius; it's designed to say whether the coolant is 80 degrees Celsius or 100 degrees Celsius.
So, don't worry about the temperature gauge being at the lowest point. Your block heater is most likely working properly nevertheless. Unless you have a far more powerful heater than I do (which is very unlikely, considering that your heater probably doesn't have a circulating pump), the temperature gauge is not supposed to display the effect of the heater.
If it's really cold outside (like -20 degrees Celsius), as you have a non-hybrid, you could always try to start the car with and without the block heater. On non-hybrid cars, starting at -20 degrees Celsius can take a second or so if you didn't use the block heater. The block heater should reduce the startup time considerably. So, you can see the effect from how easily the engine starts. Hybrid engines always start easily, so on my car I can't tell the difference.
You could always verify the operation of the heater by purchasing $10-$20 wattmeter and observing the used power. If the power is over 200 watts, your heater is probably operating normally. It is difficult to say what amount the wattmeter should show, as I don't know the specs of your block heater, but generally heaters are between 200W and 1000W, and the highest part of the range requires circulating pump for the coolant.
If you want the interior of the car to be warm, I recommend purchasing an interior heater. They are generally far more powerful than block heaters (mine is 1900W, I believe).